Apparatus for extrusion of thermoplastics

ABSTRACT

IN A METHOD AND APPARATUS FOR THE EXTRUSION OF THERMOPLASTIC RESINOUS MATERIALS AND, IN PARTICULAR, MOLTEN THERMOPLASTIC RESIN CONTAINING VOLATILE BLOWING AGENT IN ADMIXTURE THEREWITH, A SEAL IS PROVIDED FOR AN EXTRUDER TO PREVENT ESCAPE OR LEAKAGE THEREFROM F THE MOLTEN THERMOPLASTIC RESINVOLATILE BLOWING AGENT MIXTURE.

United States Patent H 1 1 ammo Johnson I 5] Jan. 15,1974

[5 APPARATUS FOR EXTRUSION OF 3,079,638 3/1963 Mille 263/329 uxTHERMOPLASTICS 3,082,816 3/1963 Skidmore 425/204 X [75] Inventor: DavidE. Johnson, Macedon,N.Y. I [73] Assignee: Mobil Oil Corporation,NewYork, Primary Examiner-R0bert Spicer,

N Y 1 Attorney-Oswald G. Hayes et al.

[22] Filed: July 14, 1971 [21] Appl. No.: 162,555

[57] ABSTRACT [52] Cl 425/207 425/4 27 In a method and apparatus for theextrusion of ther- [51] Int Cl B29) 1/06 1329f 3/02 moplastic resinousmaterials and, in particular, molten [58] Fie'm 425/205 207 209thermoplastic resin containing volatile blowing agent 264/329 inadmixture therewith, a seal is provided for an extruder to preventescape or leakage therefrom of the 56] References Cited moltenthermoplastic resin-volatile blowing agent mixt UNITED STATES PATENTS2,838,790 6/]958 Hartman 425/378 X 11 Claim, 3 Drawing Figures APPARATUSFOR EXTRUSION OF THERMOPLASTICS BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to the extrusion ofthermoplastic resinous materials and, in particular, mixtures ofthermoplastic resinousvmaterials with a volatile foaming agent. Suchmixtures are extruded to form thermoplastic foam materials. Inaccordance with the method and apparatus of the present inventiongin anextrusion system for producing such foam materials where two extrudersmay be employed, the first or primary extruder for melting the resinousthermoplastic and mixing it with the volatile blowing agent and; thesecond extruder wherein the mixture is cooled to a temperature suitablefor extrusion thereof into a foamed structure, a novel seal is providedin the area where the molten mixture from the primary extruder isintroduced into the secondary or cooling extruder whereby undesirableleakage of this mixture is minimized or substantially eliminated.

2. Description of the Prior Art In the past, in extrusion systemsemploying a primary and secondary extruder, as aforedescribed, for theproduction of thermoplastic foam materials, problems have beenencountered at the point where molten resin containing a volatileblowing agent was introduced from the primary extruder into thesecondary extruder. At this point, the material flowing from the primaryextruder is under a relatively high pressure, i.e., on the other of fromabout 500 up to about 10,000 psi. Such pressures cause a portion of themolten material entering the secondary extruder to have a tendency toflow out through the rear of the secondary extruder at a pointintermediate the terminus of the screw and the barrel surrounding theextrusion screw rather than advancing through the extruder to the outletor dieorifice end of thesecondary extruder. Attempts in the past toremedy this undesirable leakage problem centered primarily on providingelastic rings or teflon packing materials at the terminal end of thecooling extruder intermediate the rotating screw and the extruder barrelsurrounding it. Such seals were found to be unsatisfactory in that afterlimited periods of usage the seals would fail for various reasons. Forexample, in the case of elastic rings, they had a tendency to take on apermanent set after limited usage which resulted in leakage at theterminus of the secondary extruder, or in the case of the teflon packingmaterials, due to the cold-flow characteristics of the teflon materialsafter limited periods of usage, again, leakage resulted.

BRIEF SUMMARY OFTI-IE INVENTION In accord with the method and apparatusof the present invention, when a mixture of molten thermoplastic resinand volatile blowing agent is fed from a primary or mixing extruder intoa secondary extruder, leakage of the molten mixture through the endportion of the secondary extruder at a point intermediate the rotatingextruder screw and the barrel of the secondary extruder is avoided bycooling a portion of the barrel immediately surrounding the end or shankportion of the secondary extruder screw, which cooling causessolidification of that portion of the molten material which tends toadvance away from the discharge or extrusion zone of the secondaryextruder whereby the frozen or solidified thermoplastic provides a sealintermediate the terminus or end portion of the extrusion screw of thesecondary extruder and the surrounding barrel to minimize orsubstantially eliminate escape of the mol- 5 ten feed mixture from theend of the secondary extruder.

BRIEFDESCRIPTION OF THE DRAWINGS FIG. l is a schematic viewof anextrusion system in which the method and apparatus of the presentinvention may be employed. I

FIG. 2 is an enlarged fragmentary cross-section of a portion of theextrusion-system illustrated in FIG. 1.

FIG. 3 is a cross-sectional view taken on line 3-3 of FIG. 2.

DESCRIPTION OF SPECIFIC EMBODIMENTS Although the following descriptionof the method and apparatus of the present invention is particularlydirected to an extrusion system wherein two extruders, i.e., a primaryextruder being fed by a secondary extruder, are employed, it will beunderstood that the present invention may be employed in any extrusionsystem wherein molten thermoplastic material is introduced into arotating screw type extruder.

As shown in FIG. ll, illustrative of a type of extrusion system whichmay employ the novel method and apparatus of the present invention,solid thermoplastic material, e.g., in the form of pellets, may beintroduced into the hopper 11 of primary extruder 12. Primary extruder12 is a standard rotating screw type extruder wherein the thermoplasticresinous pellets are advanced by the rotation of the screw member and,during their advancement, are melted by the shearing action of theflights of the screw forcing the material against the interior of thebarrel of primary extruder 12. Additional heating means such as a heatedfluid, circulated internally of a jacket around the barrel surroundingthe screw of primary extruder 12, may be employed to promote the meltingof the thermoplastic resin therein. After the thermoplastic material hasbeen melted in primary extruder 12 a volatile liquid blowing agentmay beintroduced into the molten resinous mixture in instances where it isdesired to produce a thermoplastic foam. Such agent may be, for example,a lower alkyl hydrocarbon such as butane, pentane, isopentane and thelike or in some instances freons may be employed; After introduction ofthe volatile blowing agent into the molten resinous material in primaryextruder 12, the materials are continually mixed together in primaryextruder 12 as they are advanced towards the outlet end of extruder 12through conduit 13 and introduced into secondary or cooling extruder 15,as illustrated in FIG. ll. After introduction of the mixture of moltenresin and volatile blowing agent into extruder E5, the molten mixture isadvanced by virtue of rotation of the rotating screw 19 in extruder E5to the outlet end of extruder 115. The outlet'end of extruder 15 maybe-equipped with an annular orifice die member 16. The moltenresin-blowing agent mixture is extruder through annular die member 16 toform a foam thermoplastic tube 17, which is subsequently expandedutilizing conventional techniques, as shown in cross section in FIG. 11,to the desired diameter. During the passage of the molten mixturethrough extruder 15, it is continually being cooled to a suitableextrusion temperature by virtue of a coolant 23 circulated internallythat as the molten mixture of thermoplastic resin and volatile blowingagent is passed through conduit 13 from primary extruder 12 intosecondary extruder there is a gap 22 at the terminus or end of theextruder 15 immediately adjacent the area of entry 14 of the moltenmixture. Since the molten thermoplastic mixture is under a relativelyhigh pressure at this point, there is a tendency of the molten mixtureto flow back towards the shank of extrusion screw 19 into the area 22intermediate the end of the extrusion screw 19 and surrounding barrelmember 18. The gap in area 22 is approximately 30 mils, this being therequisite spacing of screw 19 from the inner surface of the barrel ofextruder 15. This causes undesirable leakage of a portion of the moltenmaterial through gap 22 rather than, as in the case of the major portionof the molten mixture introduced at 14, being advanced towards theextrusion end of extruder 15 to annular die member 16.

As shown in FIG. 2 and FIG. 3, in accord with the method and apparatusof the present invention this undesirable leakage of material throughgap 22 is eliminated or substantially reduced by providing that portionof the barrel of extruder 18 which surrounds the terminal, shank-end orunflighted area of extrusion screw 19 with a cooling member 25 throughwhich there is circulated a cooling fluid, such as water for example, inthe hollow annular area 24, whereby the portion of molten material inzone 22 is positively cooled by conduction. Such cooling of the shankportion of extruder screw 19, as well as cooling of the interior surfaceof member 25, results in the molten thermoplastic mass which enters zone22 to solidify. This solidification has been found to block zone 22 evenunder the extremes of pressure hereinabove discussed so that leakagethrough zone 22 of the molten mixture is either completely eliminated orsubstantially reduce, thereby forming a very effective seal. The sealthat is formed is a dynamic seal in that the frozen thermoplastic whicheffects sealing is continuously withdrawn from zone 22 by virtue ofreturn flight members 21 on extruder screw 19 in zone 22, and, as thesolidified material is withdrawn, fresh molten thermoplastic replaces itin zone 22 and is frozen by virtue of the coolant circulated throughzone 24. The solidified thermoplastic withdrawn from gap 22 iscontinuously fed by return flights 21 into the main extrusion zone ofextruder 15 and carried by forwarding flights to the outlet or extrusionzone of extruder 15 and is expressed through annular die member 16.

As shown in FIGS. 2 and 3 the screw member 19 in the area of the returnflights 21 and the unflighted shank portion of the screw ischaracterized by having a hollow annular core 26. Cooling fluid, such aswater for example, may be circulated through core 26 to further assistsolidification of molten material entering gap 22.

It will be noted that by virtue of the above-described method andapparatus, rather than using extraneous materials such as gasketing andthe like to effect seals in such extrusion systems as had beenineffectively employed by the prior art, by freezing the molten feedmaterial in the end portion of the secondary extruder, continuallywithdrawing the frozen material back into the mainstream of moltenmaterial and replacing the withdrawn materialwith new molten materialwhich is subsequently solidified, the process continuing to cycle insuch fashion throughout the entire extrusion operation, an extremelysimple, inexpensive and effective method is achieved whereby undesirableleakage of material being extruded is now completely eliminated orsubstantially reduced.

The following is a specific example which employs the method andapparatus of the present invention. This example is given by way ofillustration only and is in no way intended as limitative of the scopeof the invention.

EXAMPLE 1 Polystyrene pellets of approximately l/16 inch diameter(supplied by the Foster Grant Co. and identified as F.G.-50S) wereadmixed in a drum tumbler with a cell size control additive mixturecomprising about 0.86 percent by weight of a mixture of sodiumbicarbonate and citric acid. The cell size control additive mixture wasutilized in the extrusion process to control the cell size of theindividual cells in the final polystyrene foam product. The mixture ofthe polystyrene pellets and cell size control additive mixture wasdischarged into feed hopper ll of extruder 12 illustrated in theextrusion system of FIG. 1. The charge progressed from the feed zone ofextruder 12 to a heat plasticizing zone which was maintained at atemperature of from about 400 to about 450 F. The resulting molten masswas then progressed by the continuous rotation of the forwarding screwof extruder 12 to a blowing agent injection zone' (not shown) inextruder 12 where approximately 5 percent by weight (based on the totalweight of polystyrene) of liquid isopentane was pumped into the moltenresin at a pressure of approximately 2000 pounds per square inch, withthe temperature in the injection zone maintained at approximately 400 to450 F. The pentane-molten polymer mixture was then continuouslyforwarded to the mixing zone or terminal portion of primary extruder 12,the temperature being maintained at a range of from about 400 to about450 F. The molten mixture was subsequently forwarded through conduit 13into secondary extruder 15 where the mixture was cooled to a temperatureof about 250 to about 265 F. prior to extrusion of the molten mixturethrough tubular die 16, whereupon tube 17 of foam polystyrene wasformed. This tube was subsequently split into a sheet of foamedmaterial, which material was flexible and ideally suited for packagingcontainer applications.

As the molten mixture from extruder 12 was fed through conduit 13 atentry point 14 of secondary extruder 15 the pressure of the moltenmixture in this area was approximately 2500 psi. At the rear of extruder15, water at a temperature of F. was fed through entry port 24 into theannular orifice 24 of cooling member 25. The water continuouslycirculating in annular orifice 24 caused solidification of that portionof the molten polymer-blowing agent mixture which, upon introductionthrough conduit 13 into extruder 15, flowed into gap 22 intermediate theunflighted shank portion of screw 19 and cooling member 25 surroundingthe unflighted portion of screw 19. This solidified molten material wascontinuously withdrawn from gap 22 by return flights 21 and reintroducedinto the mainstream of the molten polymer in extruder l5 and forward bythe advancing flights 20 of screw 19 to about 12 months, showed no signsof failure.

EXAMPLE 2 Utilizing the same apparatus, feed mixture, additives,temperatures and pressures described in foregoing Example l, a moltenmixture of polymer and blowing agent was introduced into extruder 15. Nocooling member 25 was employed around the unflighted shank of extruderscrew 19 but in its place a teflon seal was positioned in gap 22 betweenthe screw and inner surface of barrel 18. It was found that after onlytwo weeks of operation, copious amounts of the molten polymerisopentaneblowing agent mixture began to escape and exude through gap 22 andthrough the end of extruder l5.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchvariations and modifications are considered to be within the purview andscope of the appended claims.

What is claimed is:

1. in an extrusion apparatus, adapted for production of thermoplasticresin foam from a mixture comprising molten thermoplastic resin andvolatile blowing agent, comprising an elongated screw disposed inside anelongated barrel, an extrusion die at one end portion of said barrel anda gap between said barrel and said screw at the opposite end portion ofsaid barrel, an inlet feed means for feeding said mixture into saidbarrel, said inlet feed means being disposed adjacent the opposite endportion of said barrel, said screw extending into said opposite endportion of said barrel and adapted for conveying the mixture fed intosaid barrel to both the extrusion die portion and said gap, and coolingmeans for solidifying, in said gap, molten feed mixture that tends toadvance from said feed inlet into said gap.

=l =l l

